Polymethyl methacrylate bone cements (PMMA bone cements) have been widely used in the field of medicine for decades for anchoring endoprostheses in the bone (Klaus-Dieter Kühn: Knochenzemente für die Endoprothetik: ein aktueller Vergleich der physikalischen und chemischen Eigenschaften handelsüblicher PMMA-Zemente (Bone cements for endoprosthetics: a current comparison of the physical and chemical properties of commercial PMMA cements). Springer Verlag Berlin Heidelberg New York, 2001). Polymethyl methacrylate bone cements consist in general of a liquid monomer component and a powder component. The liquid monomer component consists of methyl methacrylate and an activator. N,N-Dimethyl-p-toluidine is preferably used as the activator. As a rule, the powder component consists of polymethyl methacrylate or polymethyl methacrylate co-methyl acrylate, an x-ray contrast agent and a radical initiator. Zirconium dioxide and barium sulphate are commonly used as x-ray contrast agents. Dibenzoyl peroxide is preferably used as radical initiator. After mixing the monomer component and the powder component, the bone cement is hardened by radical polymerisation of the monomer within a few minutes.
After mixing, common polymethyl methacrylate bone cements are present as a white to slightly yellowish paste-like mass. As a result, an optical differentiation between the bone cement and the bone tissue causes problems from time to time when the mixed bone cement is introduced into the bone. However, it is desirable for the bone cement to be visually distinguishable without problems from the surrounding bone tissue.
For this reason, the polymethyl methacrylate bone cements which have been produced by Heraeus Kulzer GmbH for approximately 30 years have a green colour. This colour is achieved by way of a green monomer component and a green powder component. Chlorophyllin is contained as dye in both components.
In the case of the polymethyl methacrylate bone cements from Heraeus Kulzer GmbH, the chlorophyllin is dissolved in the liquid monomer component by means of refined peanut oil (Biskin®) as solubiliser. Apart from the dyed monomer components, polymethyl methacrylate bone cements can also contain a dyed powder component. A method, known as such, for dyeing the powder component of the polymethyl methacrylate bone cements consists of using dyed polymethyl methacrylate particles or polymethyl methacrylate co-methyl acrylate particles. These can be combined with a non-dyed second polymer in order to influence the characteristics of the polymethyl methacrylate bone cements. One of the problems occurring in this case involves accurately reproducing the colour, the colour impression of the powder component even in the case of different mixing ratios of the dyed polymer to the non-dyed polymer.
The synthesis of dyed polymethyl methacrylate particles or polymethyl methacrylate co-methyl acrylate particles in the course of which the dye is enclosed, during bead polymerisation, in the polymer beads being formed is highly complex and labour/time consuming under industrial conditions. A major reason for this is the occasionally low stability of dyes vis-à-vis the radical initiators used in bead polymerisation and vis-à-vis the radicals occurring during polymerisation. The initiators, in particular, can cause oxidation processes and consequently decolourise the dye.
The consistent nature of the polymethyl methacrylate bone cements, in terms of colour, is an essential factor for the acceptance of the bone cements by the customer and consequently of economic importance.